Signal integrity and system performance in digital subscriber line (DSL) systems is a function of transmission power. In other words, the signal-to-noise ratio (SNR) of a signal generally increases as the transmission power increases, and, conversely, the SNR of the signal decreases as transmission power decreases. While the power consumption at individual customer premises (i.e., upstream power consumption) is relatively small, the aggregate effect of hundreds of DSL ports in a DSL access multiplexer (DSLAM) at a central office (CO) becomes significant (i.e., the downstream power consumption becomes significant).
In the specific context of asymmetric digital subscriber line (ADSL) systems, the power level of a downstream signal to a given receiver location is determined by the ADSL transceiver unit (ATU) of the receiver (i.e., the ATU-R), and not the ATU at the CO (i.e., the ATU-C). Thus, in a conventional bit-loading approach within traditional ADSL systems, the ATU-R controls the transmission power, and the role of the CO is merely to comply with the request of the ATU-R. Since a cutback in power results in a loss of performance, the conventional bit-loading approach results in a sub-optimal use of power because the ATU-R invariably requests more power than needed for signal transmission. In other words, there is little incentive for the ATU-R to request a power cutback since a power cutback would almost always result in a loss of performance. Consequently, the CO consumes more power than necessary because the downstream signal power remains unknown until both the ATU-R and ATU-C enter data mode.
Additionally, during initialization, the entire downstream band is transmitted with a flat power spectrum (i.e., the same energy across the entire power spectrum). Since a flat power spectrum is rarely the optimum power spectrum, the initialization procedure is executed with a sub-optimal use of power. Furthermore, as loop length (i.e., reach between the CO and the customer premises) exceeds a given distance, the SNR decreases to the point where not all bins are used for data transmission. In other words, for long loops, only a portion of the bandwidth is used for data transmission. Thus, a full-bandwidth initialization of signals consumes more power than necessary for those loops that utilize only a portion of the bandwidth.
Due to the sub-optimal power consumption, which exists in ADSL systems as well as other types of signal transmission systems, there is a need in the art for reducing power consumption during signal transmission.